The objectives of this research include a comprehensive study of the mechansms of differentiation and development and the action of thyroid hormone using amphibian metamorphosis as a model system. Particular attention is paid to the initiating mechanisms by which thyroid hormones trigger the metamorphic process. Many basic facets of protein and nucleic acid biosynthesis are involved. We also hope to contribute to knowledge of comparative biochemistry, embryonic development and biochemical evolution. In this broad field, emphasis will be placed on seven major objectives: (1) The development of viable amphibian cell cultures to study the effect of hormones on intact cells, e.g. the liver, kidneys, and other tadpole tissues. (2) Studies on the increase in DNA and RNA biosynthesis in spontaneous and induced metamorphosis in numerous tissues. The DNA biosynthetic effect which is associated with mitochondrial and nuclear development may involve the metabolic life history of these important organelles. (3) Protein biosynthesis in liver and other anabolic tissues with particular reference to changes in nuclear and mitochondrial proteins. (4) Metabolism of regressing tissues, particularly in relation to the activities of particular cells which may be responsible for the increase in hydrolytic activity in these tissues. (5) Further studies on metalloprotein metabolism during metamorphosis - in particular ceruloplasmin, transferrin and hemoglobin. (6) The possibility that additional factors, hormonal or otherwise, may be involved in at least certain responses during metamorphosis. (7) A study of the ability of tadpole serum and tissue proteins to bind T3 and T4 and to relate this binding to the function of these hormones in tadpole.